doc: mention apt.txt in preparing.rst

You should mention the use of an apt.txt file in the documentation! It came handy for me to install gfortran alongside a standard Python environment defined with a requirements.txt file.

I would also add a note or mention the (advanced) possibility of using a Dockerfile in preparing.rst.

[choldgraf]

thanks for bringing these kinds of issues! Not everybody opens documentation-related issues, but they are very helpful!

[betatim]

You know what I am going to say: could you make a PR to add a section about apt.txt? The only reason it is missing is that no one has gotten around to writing it down yet.

Dockerfiles are covered in the next section of the docs. Maybe we need to rename the section titles a bit to make it more obvious that one follows after the other. Personally I am Ok with it being a bit difficult to discover that you can use a Dockerfile because it feels like in the past everyone was rushing to making their own, even when they didn’t have to. Debugging them is much harder and it is more complex to get right. So from that point of view persuading as many people as possible to not do it is a win for me. At the same time we want to know about people using Dockerfiles to see why it is they are using them so you can move things that are done across many Dockerfiles into a “supported mechanism” (like e.g. apt.txt)